The subject matter of this application is related to the subject matter of British Patent Application No. GB 0105502.9, priority to which is claimed under 35 U.S.C. xc2xa7119 and which is incorporated herein by reference.
1. Field of the Invention
Embodiments of the present invention generally relate to an apparatus and method of compensating for DC link voltage variations in an electric drive system. More particularly, embodiments of the present invention relate to the operation of a switched reluctance drive that periodically reads a value corresponding to a DC link voltage and adjusts a speed indication signal to compensate for changes in the DC link voltage. The compensated speed signal is transmitted to a controller where it is used to compensate the energization of the windings of the switched reluctance drive.
2. Description of Related Art
The general theory of the design and operation of switched reluctance machines is well known and is discussed, for example, in xe2x80x9cThe Characteristics, Design and Applications of Switched Reluctance Motors and Drivesxe2x80x9d, by Stephenson and Blake and presented at the PCIM ""93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993 and incorporated herein by reference. The general construction and operation of controllers for switched reluctance drives is generally understood and is described herein for background purposes only.
The switched reluctance motor is generally constructed without windings or permanent magnets on the moving part of the motor (called the rotor). The stationary part of most switched reluctance motors (called the stator) includes coils wound around stator poles that carry unidirectional current. In one type of switched reluctance motor, coils around opposing stator poles are connected in series or parallel to form one phase winding of a potentially multi-phase switched reluctance motor. Motoring torque is produced by applying a voltage across each of the phase windings in a predetermined sequence that is synchronized with the angular position of the rotor so that a magnetic force of attraction results between poles of the rotor and stator as they approach each other. Similarly, generating action is produced by positioning the pulse of voltage in the part of the cycle where the poles are moving away from each other. In typical operation, each time a phase winding of the switched reluctance machine is energized, magnetic flux is produced by the phase winding, thereby causing a force of attraction on the rotor poles.
A typical prior art drive is shown schematically in FIG. 1 driving a load 19. The arrangement includes a DC power supply 11 that can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is supplied across a DC link, represented as line 10, and switched across phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. A current transducer 18 is used to provide current feedback signals to the controller. In the present application, the DC voltage provided to the switched reluctance machine (whether from a battery, rectifier or otherwise) is referred to as the xe2x80x9cDC link voltagexe2x80x9d.
For proper operation of the drive, the switching must be correctly synchronized to the angle of rotation of the rotor. The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energization with respect to rotor position. Detection of rotor position is conventionally achieved by using a transducer 15, shown schematically in FIG. 1, such as a rotating toothed disk mounted on the machine rotor, which cooperates with an optical or magnetic sensor mounted on the stator. A pulse train indicative of rotor position relative to the stator is generated and supplied to control circuitry 14, allowing accurate phase energization.
In order to maintain the speed and related torque developed by a switched reluctance machine, it is desirable to control carefully the instants at which voltage is applied to the phase windings. However, changes in the voltage of the AC supply and changes in the electrical environment in which the drive system operates often result in a DC link voltage that varies over time. Because the supply voltage can vary significantly, a control scheme that ignores changes in supply voltage may experience a significant reduction in ability to control the machine as demanded by the user. This is because the flux produced by the phase windings is directly related to the applied voltage. Accordingly, a change in the supply voltage may result in more or less flux produced by the phase windings than would otherwise be desired. This undesirable change in the flux in the motor can result in degraded performance.
In most known switched reluctance drives, the relationship between the speed of the motor, the desired torque and the time for which voltage is applied to the phase windings is determined empirically through a process referred to as characterization, in which the operating parameters of the motor are determined for a wide variety of operating conditions. These operating parameters are then stored in an analog or digital circuit called a control law table.
Typically, a digital control law table is simply a series of storage locations in a memory of some sort, in which each location corresponds to a point on a control map. For that point, typical systems store an on-angle, an off-angle and possibly a free-wheel angle which will be appropriate for that operating point. These parameters may be stored in adjacent locations in the table. Alternatively, the table may be sub-divided and be thought of as a series of tables, each holding values for one of the parameters.
An example of a control law table is shown in FIG. 2. In this example, the torque is discretized into 128 values at any speed, i.e. at any chosen speed, 100% torque at that speed corresponds to the 128th storage location and 50% torque corresponds to the 64th location, etc. Likewise, the speed is discretized, but in this case into 256 values. In each location in the table are stored control parameters A for controlling the machine to provide the corresponding speed and torque. As mentioned above, each of the parameters A could be a series of control parameters or could be a single control parameter, with each location of the table containing a pointer to other tables to enable other parameters to be found. Other methods of storing the data are known in the art, for example, a sparse matrix of 16xc3x9716 locations could be used and some method of interpolation used in real-time to determine the appropriate parameter values.
FIGS. 3 and 4 show examples of control maps that correspond to control law tables. In these, the horizontal axis represents speed and extends up to a maximum of 256 units, which is representative of the maximum speed of the drive. The solid line represents the maximum torque available at any speed. Any one point on the map corresponds to a location in the control law table holding the parameter(s) required to operate at that point. The example of FIG. 3 is representative of a system with a relatively constant torque output, whereas the example of FIG. 4 is representative of a system in which the torque output changes significantly with speed.
During operation of the drive, the control system provides the control law table with signals representing the speed of the motor and the desired torque and thus finds the firing angles appropriate to that operating condition. These firing angles are used to control the switches and thus control the energization of the phase windings. The use of motor controllers with control law tables is generally understood and is not discussed herein.
In order to minimize the effort involved in the characterizing process, it is common practice to keep the DC link voltage constant during the process. Realizing that the DC link voltage will typically vary when the drive system is installed in a working environment, the characterization is sometimes done assuming that the DC voltage is at its lowest expected value. In other words, characterization is done assuming the xe2x80x9cworst casexe2x80x9d DC link voltage. This type of characterization is undesirable to the extent that it characterizes the switched reluctance drive to run in a worst-case scenario (which may seldom occur) rather than at the desired optimum DC link value, at which the efficiency of the drive would be at its highest. In addition, an increase in operating voltage may increase the available torque and current to such an extent that components of the drive may overheat and fail. Other drives may be characterized at their optimum DC link voltage. For these drives, there must be some compensation for changes in the DC link voltage during operation or the drive will suffer diminished performance, particularly at low supply voltages.
Some switched reluctance drives use an analog system to compensate for DC link voltage variations. In general, these systems compensate for the changes in the DC link voltage by adjusting the speed signal provided to the control law table so that the speed signal provided to the controller is not the actual speed of the motor but a modified speed signal that represents the actual speed signal modified by a signal representative of the DC link voltage. Other drives use digital methods of compensation for voltage variations. For example, commonly assigned U.S. Pat. No. 5,724,477 (Webster et al.), incorporated herein by reference, discloses a method of DC link compensation that uses digital sampling of the DC link voltage, using an analog to digital converter (ADC), to determine variations.
In Webster, a DC link voltage is sampled using an ADC, the sampled voltage being fed into a controller where the ratio of the characterizing voltage to the measured operating voltage is used to modify the actual rotor speed to produce a compensated speed parameter with which to interrogate the control law table. This means that the compensated speed signal is adjusted inversely to the measured DC link voltage. For example, a rise in DC link voltage produces a smaller-valued, compensated speed signal, while a fall in DC link voltage produces a larger-valued, compensated speed signal. The compensated speed signal is then applied by the controller to index the control law table and hence select appropriate firing angles.
As an example of the application of the method of Webster, if the characterization is done for a DC link voltage of 560V and the drive was being operated at 538V at a speed represented by 227 units, then by applying linear scaling to the speed signal [i.e. (560/538)*227], a compensated speed of 236 units is obtained. This compensated speed is used to interrogate the control law table for whatever level of torque is required. In this case, the change in voltage is relatively small (around 4% of the characterizing voltage). If the torque/speed profile is relatively flat in the operating region, as shown, for example in FIG. 3, the combination of these factors means that the change in torque between the actual and compensated speeds is small. For example, if the demanded torque were 80% (so that the firing angles in the storage location 80/100*128=102 are used), storage location 102 at the compensated speed will probably supply firing angles that give a torque value so close to 80% that the difference is negligible. In contrast, if the slope of the torque curve were large, such as shown in FIG. 4, the torque value corresponding to the location at the compensated speed would be very different from the torque value for the uncompensated speed. FIG. 4 shows the error in the torque for a 40% variation in voltage. This makes the prior art compensation methods unsuitable in many circumstances.
While the above-described systems can work well over a limited range of voltage variation, depending on the form of the torque/speed curve, they do not give good results when widely varying voltages are encountered. Such circumstances arise particularly in systems not connected to the public utility supply, e.g. in aerospace, marine or automotive environments, where voltage variations of over 50% are not uncommon. While these variations could be partially catered for by characterizing the system at different voltages, this is expensive in development time, requires considerably more storage space in the control system, makes reading the control laws more difficult and, at best, gives accurate compensation only at certain voltages.
Given the disadvantages of known compensation systems, it is desirable to have a DC link voltage compensation system that is accurate, covers a wide range of DC link voltages and is relatively inexpensive. Embodiments of the invention achieve these and/or other advantages.